Computed tomography (CT) equipment has gained widespread acceptance as medical equipment. The CT equipment comes in various types: X-ray CT, nuclear magnetic resonance (NMR) CT, positron CT and ultrasonic wave CT. The most recent version of the equipment is optical CT equipment that utilizes visible or near infrared light.
The optical CT equipment operates on the so-called back projection method for image reconstitution. To use this method requires procuring projection data which is obtained as follows: a light beam is first irradiated to various points on the test subject to measure the intensity of the transmitted light from the subject in diverse directions. The measurements are used to find the light transmittance of the subject at the respective irradiation points. To acquire the projection data further involves having the irradiation position of light scan a number of points along the periphery of the subject under test; at each of the irradiation points, the light transmittance of the subject must be measured in various directions.
The conventional scanning methods for irradiating the light beam to the test subject are typically shown in FIGS. 2 and 3. The method of FIG. 2 is disclosed in Japanese Patent Laid-Open No. 115548/1988. This method involves retaining a light source 21 and a photo detector 22 within the same gantry 23 that is rotated around the test subject 20. The rotation of the gantry 23 changes the irradiation position of light and the detecting position of the transmitted light. The method of FIG. 3 is disclosed in Japanese Patent Laid-Open No. 56411/1989. According to this method, a light beam guided from the light source 31 via an optical fiber 32 is made to scan a test subject 30. The scanning is accomplished by use of an optical scanner 33 comprising a wobbling mirror 33a. The light transmitted through the subject 30 is detected by a photo detector array 34 having numerous photo detectors arranged therein. The individual photo detectors in the photo detector array 34 are switched electrically to vary that point on the subject at which to measure the transmitted light.
Another conventional method is illustrated in FIG. 4. This method, disclosed in Japanese Patent Laid-Open No. 72542/1985, employs optical fibers 43 and 44 in switching between two positions: irradiation position on the test subject 40, and measuring position of the transmitted light from the subject 40. One disadvantage of the methods in FIGS. 2 and 3 is, as illustrated, their bulky equipment attributable to the relatively large mechanics required. The method of FIG. 4 is an attempt to bypass that disadvantage using optical fibers.
Without direct relevance to the optical CT, there exists a conventional technique for consecutively monitoring a plurality of measuring points. This technique, disclosed in Japanese Patent Laid-Open No. 276000/1986, utilizes light for transmission of information from the sensors at the measuring points to a central monitoring unit. The technique also uses an optical system comprising optical fibers and a dovetail prism arrangement for switching between the transmission and the reception of light to and from the sensors and the central monitoring unit.
The method of FIG. 4 appears to have resolved one of the disadvantages of the prior art in that it simplifies the mechanics through the use of optical fibers. In fact, the method has created a number of new problems. One such problem is that the method has low efficiency in utilizing the light coming from a light source 41. The reason for the reduced efficiency is as follows: the thread ends on one side of an optical fiber 43 are located in individual irradiation positions 45 oriented toward the test subject 40, and the thread ends on the other side of the optical fiber 43 are arranged in a circular manner on a fiber holder 46. The light source 41 is placed in front of the fiber holder 46. The light from the light source 41 is made incident on the optical fiber 43 so that the light will be expanded for irradiation onto the entire surface of the holder 46. At this point, a rotary disk 42 having a single hole therethrough is placed before the fiber holder 46. Rotating the rotary disk 42 allows the light to go through the hole into each of the optical fiber threads successively. Only the light having passed through the hole enters the optical fiber 43. This scheme of consecutively switching the irradiation position over the test subject 40 has the disadvantage of getting the rotary disc 42 to block most of the light coming from the light source 41; only a limited part of all light from the light source 41 is utilized for irradiation to the subject 40. This means that to obtain a sufficient quantity of light for actual measurement requires furnishing a light source of a very large output.
Another disadvantage of the method of FIG. 4 is as follows: the use of a single photo detector 47 entails a very wide range of the intensity of the light that is incident on the detector. That is, the photo detector 47 must be one having an extensive dynamic range. The method of FIG. 4 involves having the single photo detector 47 detect on a time series the light transmitted through various measuring points on the periphery of the subject 40 via the optical fiber 44. The quantity of the transmitted light varies considerably from one measuring point to another; the variation in light intensity can span several orders of magnitude. The photo detector 47 thus needs to be one that is expensive and provides a dynamic range addressing several orders of magnitude.